Re: Telephone connected to NBN disconnects at 3 hours
I did say that Optus Helpdesk is pretty variable. You could call back today and possibly get a completely different answer.
What model landline phone do you have? It is possible there's something in that model (as other phones I googled can have max call duration settings.)
Good Luck with the TIO. They are mainly concerned with enforcing contracts (which in this case appears to be being met) Perhaps they'll find someone who knows the answer to your question though. You should make an official complaint to Optus first (I suggest write) as the TIO tends to require a complaint number before proceeding.